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Let's say there's a stainless steel bar pointing to the sky from the ground. Like this there's a possibility it gets struck by lightning.

Now would putting a rubber cap on top of the bar change this or would the electricity travel around that cap and still hit the bar?

If not, would the steel bar be fully protected against lightning strikes like this?

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The electric field in the atmosphere will induce charges on the bar or the rubber cap. The surface density of the induced charges depends on, amongst other things, the curvature of the rod/rubber surface. Edges and points have a higher surface charge density than flat surfaces. The greater the surface charge density the greater is the likelihood of a lightning strike. The chances are that the rubber cap will have edges which are less sharp and so the probability of a lightning strike is reduced but I doubt that it will be by very much because the lightning can bypass the rubber and just head for the metal.
Rubber is also an insulator but if it is wet then the layer of surface moisture will act as a conductor. Even if there was no water present and the rubber was dry given the size of the electric field which will be present the rubber will probably still conduct.

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  • $\begingroup$ A sharp point serves to attract lightning strikes; a sphere, OTOH, serves to diffuse the charge in the air, and thus avoids lightning strikes. Thus you want a sphere at the tip of the lightning rod on your barn, but if you have a nearby hill, put a spike at the tip in order to divert the lightning strikes to the hill. $\endgroup$ – Peter Diehr Mar 1 '16 at 18:03

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